Motor vehicles with diesel engines are gaining an ever greater share in the entire vehicle stock, inter alia, because of their comparatively low fuel consumption values. This also applies to diesel passenger cars, in particular.
A major problem of diesel engines, however, continues to be the purification of their exhaust gas. In addition to the pollutants known from gasoline engines, i.e., carbon monoxide (CO), hydrocarbons (HC) and various nitrogen oxides (NOx), the exhaust gas of diesel engines also contains soot particles. Moreover, diesel engines are operated with a lean air/fuel mixture so that their exhaust gas contains a high proportion of oxygen of about 5 to 15 vol.-%, while the exhaust gas of stoichiometrically operated gasoline engines has an oxygen content of only about 0.7 vol.-%. As a rule, the exhaust gas of diesel engines is also substantially colder than that of gasoline engines.
The exhaust gas of modern diesel engines for passenger cars has temperatures between only about 80 and 25° C. in urban traffic, i.e., when operated at partial load. Exhaust-gas temperatures reach 400 to 500° C. only at full load. These numbers refer to the temperatures at the engine outlet and, respectively, downstream of a turbocharger widely used today. Due to the thermal losses along the exhaust-gas purification system by heat dissipation and heat conduction the exhaust-gas temperature upstream of a catalyst arranged in the underfloor area lies substantially below these values.
These particular features of the diesel exhaust gas pose corresponding problems during its purification. Thus, the high oxygen content makes it impossible to simultaneously convert carbon monoxide, hydrocarbons and nitrogen oxides into the harmless compounds of water, carbon dioxide and nitrogen as in a stoichiometrically operated gasoline engine. However, as the emission of nitrogen oxides from a diesel engine is usually low it is frequently sufficient to purify the diesel exhaust gas using a so-called diesel oxidation catalyst, that is, carbon monoxide and hydrocarbons are converted into water and carbon dioxide at the diesel oxidation catalyst. Catalyst suitable for this purpose are described in DE 39 40 758 A1 (U.S. Pat. No. 5,157,007), U.S. Pat. No. 5,928,981 and EP 0 920 913 A1 (U.S. Pat. No. 6,342,465 B1), for example. These catalysts usually comprise a platinum-activated aluminum oxide mixed with other oxidic components, such as silicon dioxide, titanium oxide and various zeolites, for example.
In order to obtain a purifying effect with such an oxidation catalyst which starts as rapidly as possible the oxidation catalyst is usually arranged closely downstream of the engine outlet.
So-called diesel particulate filters are employed to remove the particles from the exhaust gas of diesel engines. These can be deep-bed filters, such as metal foams, ceramic foams or fiber filters, or surface filters. The so-called wall flow filters are suitable for use as surface filters. These are filters of the ceramic honeycomb body type, as are used in large numbers as carriers for catalytically active coatings in exhaust-gas catalysis. The entry and discharge openings of the flow passages of these honeycomb bodies are alternately closed to achieve a filtering effect so that the exhaust gas is forced to flow through the porous walls of the flow passages on its way through the wall flow filter. In doing so, the soot contained in the exhaust gas deposits mainly on the walls of the flow passages.
Increasing soot deposition continuously raises the exhaust backpressure of the filter. Therefore, the filter must be regenerated from time to time by burning the soot off. This, however, requires temperatures of the particulate filter of at least 600° C. in order to ignite the soot combustion. By coating the filter with a so-called soot ignition coating the ignition temperature of the soot can be lowered by about 100 to 150° C. But even in this case other active modifications are still necessary to raise the exhaust-gas temperature at the site of the particulate filter to the ignition temperature.
There exist various concepts for raising the temperature of the particulate filter. In any case, however, the regeneration of the filter entails an increase in energy or fuel consumption. The temperature of the filter may be raised by internal engine modifications, such as re-injection of fuel, late combustion point, multistage combustion or by external heating.
Internal engine modifications lead to an increase of the engine's exhaust-gas temperature. At the same time, the content of unburned hydrocarbons in the exhaust gas rises. These hydrocarbons are burned on the oxidation catalyst usually provided upstream of the particulate filter. The heat of combustion released in this process increases the exhaust-gas temperature further. It is also known to directly inject fuel into the exhaust gas upstream of the oxidation catalyst and burn it at the catalyst.